In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Plaintext has reference to the operation of cryptographic algorithms, usually encryption algorithms, and is the input upon which they operate. Cleartext, by contrast, refers to data that is transmitted or stored unencrypted (that is, 'in the clear').

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With the advent of computing the definition of plaintext expanded to include any data, including binary files, in addition to simple messages and human-readable documents, in a form that can be interpreted or used without needing to be processed using information not generally available (a key). The information, which would normally be called a message, document, file, etc., if to be communicated or stored in encrypted form is referred to as plaintext.

Plaintext is used as input to an encryption algorithm; the output is usually termed ciphertext, particularly when the algorithm is a cipher. Codetext is less often used, and almost always only when the algorithm involved is actually a code. In some systems multiple layers of encryption are used, with the output of one encryption algorithm become the "plaintext" input for the next.

Weaknesses can be introduced into a cryptosystem through insecure handling of plaintext, allowing an attacker to bypass the cryptography altogether. Plaintext is vulnerable in use and in storage, whether in electronic or paper format. Physical security deals with methods of securing information and its storage media from local, physical, attacks, for instance by entering a building and gaining access to papers, storage media, or computers. Discarded material, if not disposed of securely, may be a security risk; even shredded documents and erased magnetic media can often be reconstructed with sufficient effort.

If plaintext is stored in a computer file , the storage media, the computer and its components, and all backups must be secure. Sensitive data is sometimes processed on computers whose mass storage is removable, in which case physical security of the removed disk is separately vital. In the case of securing a computer, useful (as opposed to handwaving) security must be physical (e.g., against burglary, brazen removal under cover of supposed repair, installation of covert monitoring devices, etc.), as well as virtual (e.g., operating system modification, illicit network access, Trojan programs, ...). The wide availability of keydrives, which can plug into most modern computers and store large quantities of data, poses another severe security headache. A spy (perhaps posing as a cleaning person) could easily conceal one and even swallow it, if necessary.

Discarded computers, disk drives and media are also a potential source of plaintexts. Most operating systems do not actually erase anything — they simply mark the disk space occupied by a deleted file as 'available for use', and remove its entry from the file system directory. The information in a file deleted in this way remains fully present until overwritten at some later time when the operating system reuses the disk space. With even low-end computers commonly sold with many gigabytes of disk space and rising monthly, this 'later time' may be months later, or never. Even overwriting the portion of a disk surface occupied by a deleted file is insufficient in many cases. Peter Gutmann of the University of Auckland wrote a celebrated 1996 paper on the recovery of overwritten information from magnetic disks; areal storage densities have gotten much higher since then, so this sort of recovery is likely to be more difficult than it was when Gutmann wrote.

Also, independently, modern hard drives automatically remap sectors that are starting to fail; those sectors no longer in use will contain information that is entirely invisible to the file system (and all software which uses it for access to disk data), but is nonetheless still present on the physical drive platter. It may, of course, be sensitive plaintext. Some government agencies (e.g., US NSA) require that all disk drives be physically pulverized when they are discarded, and in some cases, chemically treated with corrosives before or after. This practice is not widespread outside of the government, however. For example, Garfinkel and Shelat (2003) analyzed 158 second-hand hard drives acquired at garage sales and the like and found that less than 10% had been sufficiently sanitized. A wide variety of personal and confidential information was found readable from the others. See data remanence.

Laptop computers are a special problem. Laptops containing secret information, some perhaps in plaintext form, belonging to the US State Department, Department of Defense, and the British Secret Service have been stolen or lost. Announcements of similar losses are becoming a common item in news reports. Appropriate disk encryption techniques can safeguard data on misappropriated computers or media.

On occasion, even when the data on the host systems is itself encrypted, the media used to transfer data between such systems is nevertheless plaintext due to poorly designed data policy. An incident in October 2007 in which HM Revenue and Customs lost CDs containing the records of 25 million child benefit recipients in the United Kingdom — the data apparently being entirely unencrypted — is a case in point.

Modern cryptographic systems are designed to resist known plaintext or even chosen plaintext attacks and so may not be entirely compromised when plaintext is lost or stolen. Older systems used techniques such as padding and Russian copulation to obscure information in plaintext that could be easily guessed, and to resist the effects of loss of plaintext on the security of the cryptosystem.

Several popular web browsers which offer to store a user's passwords do so in plaintext form. Even though most of them initially hide the saved passwords, it is possible for anyone to view all passwords in cleartext with a few clicks of the mouse, by going into the browsers' security settings options menus. In 2010, it emerged that this is the case with Firefox (still the case as of end-2014), and in Aug 2013 it emerged that Google Chrome does so as well.[1] When a software developer raised the issue with the Chrome security team,[2] a company representative responded that Google would not change the feature, and justified the refusal by saying that hiding the passwords would "provide users with a false sense of security" and "that's just not how we approach security on Chrome".[3]